According to the studies by the inventors of the present invention, a circuit receiving a radio signal (hereinafter, referred to as a receiver) is a circuit which performs frequency conversion to a lower frequency and amplification to a desired level without reducing a signal-to-noise ratio (hereinafter, abbreviated to SNR) of the signal inputted from an antenna and distorting the signal as far as possible. For this reason, the receiver is provided with a low-noise amplifier circuit (hereinafter, referred to as LNA) disposed at a position as close as possible to an antenna.
The LNA is required to have such features as (1) high gain, (2) a ratio between a signal-to-noise ratio at an LNA input and a signal-to-noise ratio at an LNA output (hereinafter, referred to as a noise factor) is small, and (3) low distortion. Further, since an efficient power transfer is necessary in order to receive a weak radio signal inputted from an antenna, the LNA is required to have (4) impedance matching between the input and the output.
However, since a mixer, an amplifier circuit and others are provided on a latter stages of the LNA in a receiver, when a strong input signal is inputted from an antenna, if the gain of the LNA is high [(1) mentioned above], the signal is distorted in the mixer, the amplification circuit and others on the latter stages, and the desired receiving operation cannot be performed. Therefore, the LNA is also required to perform the gain switching operation in general.